Alignment Layer with Reactive Mesogens for Aligning Liquid Crystal Molecules

ABSTRACT

The invention relates to an alignment layer with improved adhesion to liquid crystal (LC) films, to a precursor material used for the preparation of such a layer, to a laminate comprising such a layer and at least one LC polymer film, and to the use of the alignment layer and the laminate for optical, electrooptical, decorative or security uses and devices, wherein the alignment layer and the precursor material comprise at least one reactive mesogen in monomeric, oligomeric or polymeric form.

FIELD OF INVENTION

The invention relates to an alignment layer with improved adhesion toliquid crystal (LC) films, to a precursor material used for thepreparation of such a layer, to a laminate comprising such a layer andat least one LC polymer film, and to the use of the alignment layer andthe laminate for optical, electrooptical, decorative or security usesand devices.

BACKGROUND AND PRIOR ART

Polymerisable liquid crystal (LC) materials are commonly used for thepreparation of optical films in liquid crystal displays. These materialsusually contain a certain amount of compounds with two or morepolymerisable groups (di- or multi-functional), which are crosslinked togive a hard film.

However, during polymerisation certain polymerisable materials, like forexample acrylates, suffer polymer shrinkage [see R. A. M. Hikmet, B. H.Zwerver and D. J. Broer Polymer (1992), 33, 89]. This shrinkage places alot of strain in the polymerised film and acts to reduce the adhesionbetween the film and the substrate.

One technique reported in prior art to overcome this problemconcentrates on modifying the substrate to improve its adhesion to thepolymerised film. For example, the substrate can be subjected to specialtreatment, for example flame treatment as disclosed in U.S. Pat. No.2,795,820 or GB 0 788 365, corona treatment as reported in DE 195 80301, or plasma treatment as reported in R. L. Bersin Adhesives Age(1972) 15, 37.

Alternatively, a separate adhesion or coupling layer (typically asolution of organo-silane materials) can be coated onto the substrate tohelp increase adhesion of a polymer film to a substrate, like e.g. thecommercially available Addid 900® ( (from Wacker GmbH, Burghausen,Germany), an amino-functional trimethoxy silane.

U.S. Pat. No. 5,631,051 discloses a method of preparing an opticalcompensation sheet on a transparent substrate of triacetyl cellullose(TAC), by first providing an adhesion layer of gelatine on the TAC film.Then an aligning layer is formed by coating a solution of denaturatedpolyvinyl alcohol (PVA), which was chemically modified by addition ofpolymerizable groups, onto the gelatine layer, evaporating the solventand rubbing the surface of the polymerised PVA layer unidirectionally,Finally an optically anisotropic layer comprising discotic LC materialis coated onto the rubbed surface of the PVA layer and polymerised.

U.S. Pat. No. 5,747,121 discloses a method of preparing an opticalcompensation sheet by coating a solution of denaturated polyvinylalcohol (PVA), which was chemically modified by addition ofpolymerizable groups, onto a transparent substrate, evaporating thesolvent and rubbing the surface of the PVA layer unidirectionally. Thenan optically anisotropic layer comprising discotic LC material is coatedonto the rubbed surface of the PVA layer and polymerised. Afterwards thefilm is subjected to heat treatment whereby the PVA layer and thediscotic LC layer are reported to be chemically bonded to each other viafree, crosslinkable groups.

However, all of the above methods have a distinct disadvantage in thatthey involve an extra processing step. Furthermore, the use of separateadhesion or aligning layers comprising isotropic materials like gelatineor PVA can negatively influence the performance of the liquid crystalfilm when used e.g. as optical film.

To overcome this problem an adhesion promoter can be directly added tothe polymerisable LC material formulation. Addid 900®, or a similarmaterial, is a typical additive. However, when these materials are addedto polymerisable LC mixtures it is often difficult to obtain wellaligned films, such as those required for optical films.

In particular films or coatings of polyimide (PI), which are commonlyused as alignment layers to induce uniform orientation of the LCmaterial provided thereon, often have only low adhesion to thepolymerised LC material.

Therefore, there is a need for an advantageous method to improve theadhesion of a film or coating, which is used as alignment layer for LCs,to an LC polymer film. The method should not negatively affect theoptical and mechanical properties of the alignment layer, like itstransmission and its stability against temperature, mechanical stressand solvents.

It was an aim of the present invention to provide such a method. Anotheraim of the present invention was to provide an alignment layer havingimproved adhesion whilst not affecting its optical and mechanicalproperties. Other aims are immediately evident to the expert from thefollowing description.

The inventors of the present invention have found that the abovementioned drawbacks of prior art methods can be overcome by using analignment layer that is obtainable from a precursor material comprisinga low amount of a polymerisable mesogenic compound (reactive mesogen).This compound improves adhesion of the alignment layer, in particularwhen used as substrate or alignment layer for an LC polymer providedthereon, whilst not negatively affecting its optical and mechanicalproperties.

Definition of Terms

The term ‘film’ includes self-supporting, i.e. free-standing, films orlayers of material that show more or less pronounced mechanicalstability and flexibility, as well as coatings or layers on a supportingsubstrate or between two substrates.

The term ‘liquid crystal or mesogenic material’ or ‘liquid crystal ormesogenic compound’ includes materials or compounds comprising one ormore rod-shaped, board-shaped or disk-shaped mesogenic groups, i.e.groups with the ability to induce liquid crystal phase behaviour. Liquidcrystal (LC) compounds with rod-shaped or board-shaped groups are alsoknown in the art as ‘calamitic’ liquid crystals. Liquid crystalcompounds with a disk-shaped group are also known in the art as‘discotic’ liquid crystals. The compounds or materials comprisingmesogenic groups do not necessarily have to exhibit a liquid crystalphase themselves. It is also possible that they show liquid crystalphase behaviour only in mixtures with other compounds, or when themesogenic compounds or materials, or the mixtures thereof, arepolymerised.

For the sake of simplicity, the term ‘liquid crystal’ (abbreviated LC)material is used for both liquid crystal materials and mesogenicmaterials, and the term ‘mesogen’ is used for the mesogenic groups ofthe material.

The term ‘reactive mesogen’ (RM) means a polymerisable mesogeniccompound.

The term ‘non-mesogenic compound or material’ means a compound ormaterial that does not contain a mesogenic group as defined above.

SUMMARY OF THE INVENTION

The invention relates to an alignment layer, preferably a solventprocessed film, which is suitable for aligning liquid crystal (LC)molecules, characterized in that it comprises at least one reactivemesogen (RM) in monomeric, oligomeric or polymeric form.

The invention further relates to an alignment layer, preferably asolvent processed film, which is suitable for aligning liquid crystal(LC) molecules, characterized in that it is obtainable from a precursormaterial comprising at least one reactive mesogen (RM).

The invention further relates to a precursor material comprising atleast one RM as described above and below.

The invention relates to the use of an alignment layer as describedabove and below as substrate and/or alignment layer of liquid crystal(LC) materials, in particular of polymerisable or polymerised LCmaterials.

The invention further relates to a laminate comprising an alignmentlayer as described above and below and further comprising a filmcomprising polymerised or crosslinked LC material.

The invention further relates to a method of preparing a laminate asdescribed above and below by.

-   -   providing a layer of a polymerisable LC material onto an        alignment layer as described above and below,    -   optionally aligning the LC material into uniform orientation,    -   polymerising the LC material.

The invention further relates to the use of a precursor material, analignment layer or laminate according to the present invention inoptical, electrooptical, information storage, decorative and securityapplications.

The invention further relates to an optical component or devicecomprising a precursor material, alignment layer or laminate accordingto the present invention.

The invention further relates to a liquid crystal display comprising aprecursor material, alignment layer or laminate according to the presentinvention.

The invention further relates to an authentification, verification orsecurity marking or a coloured image comprising a precursor material,alignment layer or laminate according to the present invention.

The invention further relates to an object or document of valuecomprising an authentification, verification or security marking or animage as described above and below.

DETAILED DESCRIPTION OF THE INVENTION

For the alignment layers according to the present invention in principleall polymer films or substrates can be used that are known to theexpert. and are suitable as aligning layers. Especially preferred aresolvent processed films, for example solutions of polyimide or films ofTAC cast from an organic solution, e.g. in dichloromethane.

Solvent processing is typically carried out by depositing a solution ofthe polymer that serves as aligning material, like for example TAC orpolyimide, dissolved in a suitable sovent, for example an organicsolvent like dichloromethane, and slowly evaporating off the solvent toproduce a film of the polymer which is not under stress and so hasvirtually no birefringence. This method is known to the expert anddescribed in the literature, for example in JP 08-258065 A.

The alignment layer according to the present invention is characterizedin that it comprises a limited amount of reactive mesogens (RMs), whichimprove the adhesion of a subsequent LC layer coated onto the alignmentlayer. However, the present invention does not include alignment layersor precursor materials that do essentially consist of RMs or LCpolymers, like for example those disclosed in WO 02/44801 or thereferences cited therein.

Thus, the amount of RMs in an alignment layer or precursor materialaccording to the present invention is generally less than 50 wt. %,preferably less than 20 wt. %, very preferably less than 10 wt. %, mostpreferably less than 5 wt. %. The birefringence Δn of an alignment layeror a precursor material according to the present invention is preferablyless than 0.05, very preferably less than 0.01, more preferably lessthan 0.005, most preferably less than 0.001.

Further preferred is an alignment layer or precursor material which,before addition of the reactive mesogens, is non-mesogenic or opticallyisotropic or only slightly optically anisotropic. ‘Slightly opticallyanisotropic’ means that the birefringence Δn is <0.01.

The alignment layer does not necessarily have to be a polymer film. Itis also possible for example to use a self-assembled monolayer ormultilayer as alignment layer.

Particularly preferred are the following alignment layers:

(1) Polyimide films obtained from a solution of a polyimide precursor,for example a solution comprising monomers and/or oligomers which uponpolymerisation e.g. by heating yield a polyimide film, especiallypreferably wherein the precursor solution comprises at least one RM ofthe following formulae

wherein

-   -   P¹, P² and P³ are independently of each other a polymerisable        group,    -   Z¹ and Z² are independently of each other, —O—, —S—, —CO—,        —COO—, —OCO—, —O—COO—, —OCH₂—, —CH₂O—, —CH₂CH₂—, —C═C—,        —CH═CH—COO—, —OCO—CH═CH— or a single bond,    -   Z³ and Z⁴ are independently of each other —COO—, —OCO—,        —CH₂CH₂—, —CH₂—, —OCH₂—, —CH═CH—, —CF═CF—, —C═C— or a single        bond,    -   Z⁵ and Z⁶ are independently of each other —O—, —COO—, —OCO—,        —CH₂CH₂—, —CH₂O—, —OCH₂— or a single bond,    -   Y¹ and Y² are independently of each other a polar group,    -   R¹ and R² are independently of each other an unpolar alkyl or        alkoxy group,    -   A, B, C and D are independently of each other 1,4-phenylene that        is optionally mono- di or trisubstituted by L¹, L², L³, L⁴, L⁵,        L⁶ or 1,4-cyclohexylene,    -   L¹, L², L³, L⁴, L⁵ and L⁶ are independently of each other H, F,        Cl, CN or an optionally halogenated alkyl, alkoxy,        alkylcarbonyl, alkoxycarbonyl or alkoxycarbonyloxy group with 1        to 7 C atoms,    -   r is0, 1, 2, 3 or 4,    -   x and y are each independently an integer from 1 to 12,    -   z is 1, 2 or 3, and    -   g¹,g²,g³ and g⁴ are independently of each other a single bond,        —O—, —COO— or —OCO—.

(2) Solvent cast polyimide films having the general formula A

which in a preferred embodiment do also comprise one or more RMs offormula I-VI as defined above.

(3) Polymer films which are effectively plasticized with one or more RMsas defined above and below, in particular one or more RMs of formulaI-VI.

‘Plasticized’ means that the RM(s) is(are) added to the polymer, similarto adding a plasticizer, to improve processability of the polymer, e.g.by lowering Tg, reducing stiffness and improving processability.

This is in contrast to the films of preferred embodiment (1) above,where the RMs are added to the polymer precursor before polymerisation.

(4) Solvent cast cellulose based films, like for example triacetatecellulose (TAC) or diacetate cellulose (DAC) films, which preferablycomprise one or more RMs of formula I-VI.

(5) Command layers comprising one or more compounds selected fromphotochromic or isomerisable compounds, chromophores and dyes, whereinchanges of the chemical structure and/or the orientational directions ofthese compounds induce a specific alignment, or change the originalalignment, of an LC material coated onto said layer. These layerspreferably comprise one or more RMs of formula I-VI dispersed throughoutthe layer.

These command layers are typically not polymer layers, butself-assembled monolayer or multilayers. Thus, the promotion ofalignment of LC material by a command layer is typically not a bulkeffect, but a surface effect, where the command layer molecules aretethered to the surface and are usually only a monolayer thick.

Suitable and preferred photochromic or isomerisable compounds in thecommand layer are for example non-polymerisable monomeric compunds thathave been modified to bind to the substrate by either chemical (e.g.siloxane/glass, thiol/gold) or physical (e.g. hydrogen bonding) bondsand which also interact with polarised UV to yield a command layer. Forexample, when irradiated with polarised UV light, layers composed ofthese materials can promote a specific alignment of LC molecules.

Such compounds are known in the art. Examples of suitable and preferredcompounds include derivatives of azobenzene, stilbenes, spiropyran,spirooxadines, α-hydrazono-β-ketoesters, cinnamate, retinylidene,chalcone, coumarins, benzylidenephthalimidines,benzylideneacetophenones, diphenylacetylene or stilbazoles [as describedfor example in the review article by K. Ichimura Chemical Reviews(2000), 100, 1847.

Especially suitable and preferred monomeric compounds, which are alsodescribed in the above article of Ichimura, are for example azocompounds of the following formulae

wherein R is an alkyl group and T is a group used to bind the moleculeto the surface, which typically comprises a flexible spacer group, likean alkyl chain that is terminated with a specific group for binding tothe surface, like for example a siloxane or —SH group.

For example, a command layer comprising one or more compounds of formulaVII after irradiation with polarised light promotes in-plane alignmentof LC molecules, whereas a command layer comprising one or morecompounds of formula VIII after irradiation with polarised lightpromotes homeotropic alignment.

Preferred alignment layers according to this invention are polyimide(PI) films obtained from polyimide precursors. These films have thefollowing advantages: (i) it is relatively simple to incorporate the RMinto the precursor solution, (ii) dissolving both the RM and precursorsin the same solution offers a simple method of premixing the materialsbefore imidization, thus providing greater entanglement of the RM in thepolyimide than obtained by mixing the polyimide and the RM together,(iii) polyimide precursor solutions are widely used in the displayindustry.

An alignment layer according to the present invention is characterizedin that one or more RMs, preferably of formula I-VI, are incorporatedinto the precursor material of the layer before it is processed orpolymerised, e.g. in case of polyimide films by adding the compoundsinto the pre-imidized polyimide solution. This requires RMs which aremiscible with the substrate materials, such that they are well dispersedthroughout it. When the polymer layer is subsequently being formed theRM is physically trapped. This provides an alignment layer whichcontains several reactive sites and improved adhesion between thealignment layer and a polymerised LC material coated thereon, inparticular if the polymerised LC material is obtained from RMs that aresimilar to those of formula I-VI above.

As precursor for the alignment layer in principle any precursor known inprior art can be used. These materials are commercially available in abroad variety, for example polyimide precursors are available from JSRCo. (Japan).

Suitable precursor materials for polyimide films are for examplenon-photosensitive precursors, the typical chemical structure of whichis represented by the following formula B

or photosensitive precursors, the typical chemical structure of which isrepresented by the following formula C

For some practical applications the polyimide precursor may also possessdifferent lateral substituted groups, in order to improve solubility ofthe material or to influence the surface energy of the material. Forexample a polyimide precursor comprising perfluorocarbon chains ispreferred for polyimides used to help promote homeotropic alignment ofliquid crystals. The structures above are given only as general examplesto help explain this invention and are not meant to restrict thisinvention.

Especially preferred precursor materials for this invention arenon-photosensitive polyimide precursors.

The preferred range of concentration of the RMs, in particular thecompounds of formula I-VI, in the precursor material of the alignmentlayer is from 0.5 to 4%, preferably from 1% to 2% by weight.

Processing of a polymer alignment layer according to the inventioncomprising one or more RMs is typically carried out by providing asolution of the polymer and the RM onto a substrate, which is heated andbaked to remove excess solvent and allowed to cool to ambienttemperature. The polymer can then be subjected to mechanical treatment,like rubbing, to provide a preferred orientation direction for the LCmaterial coated thereon.

A preferred process relates to spincoating a solution of polyimideprecursors and one or more RMs of formula I-VI onto clean glass slides,heating the slides, for example at approx. 100° C. for several tens ofseconds to one or 2 minutes, in order to remove excess solvent andbaking the slides, for example at approx. 180° C. for several tens ofminutes to 1 to 2 hours, in order to imidize. The slides are thenallowed to cool to ambient temperature and the resulting polyimide layeris rubbed for example with a velvet cloth.

The RMs should be miscible with the polymer. Furthermore, thetemperatures of processing the polymer film should not destroy thepolymerisable group of the RM.

The RMs can be present in monomeric, oligomeric or polymeric form in thefinal alignment layer.

The term ‘oligomer’ in prior art usually means a compound with a smallnumber of monomeric or repeating units, whereas the term ‘polymer’usually refers to a compound having a higher number of monomeric orrepeating units. The boundaries between these terms are usually notfixed. In the sense of this invention, ‘oligomeric’ means a compoundhaving at least 2, but not more than from 15 to 50, preferably not morethan from 20 to 30, very preferably not more than 25 monomeric orrepeating units, and the term ‘polymeric’ means a compound having ahigher number of monomeric or repeating units than that defined for theoligomeric compound. These repeating units can be identical or, e.g. incase of a copolymerisate of different RMs, can be different.

Preferably processing of the alignment layer material is carried outsuch that after preparation of the alignment layer there are still someunreacted polymerisable groups in the RM additive which can then reactwith a subsequent LC layer coated onto the alignment layer.

When the RMs are polymerised, for example during baking of the alignmentlayer, the number of non-reacted polymerisable groups relative to therepeat unit of the mesogenic core of an RM decreases with increasingmolecular weight of the polymer formed by the RMs. On the other hand,the chance of entanglement of the RM additive in the polymer of thealignment layer increases. Therefore monomeric and oligomeric forms ofthe RMs are especially preferred, whereas polymeric forms are possiblebut less preferred.

Preferably, the RMs remain non-polymerised throughout the alignmentlayer processing and are only polymerised when a subsequent LC layercoated onto the alignment layer is polymerised. In this way the RMadditive is entangled in the material forming the alignment layer, andcan then be polymerised (chemically bound) to the subsequent LC layer.However, it is also possible that the RM additive undergoespolymerisation to a certain degree within the alignment layer before thesubsequent LC layer is added.

Especially preferably the RM(s) is(are) present in monomeric oroligomeric form in the alignment layer after its preparation, i.e.before a subsequent LC layer is coated onto the alignment layer.

In case a polymerisable LC material is coated onto the alignment layer,the unreacted RMs can then be copolymerised with the polymerisable LCmaterial, for example according to known methods that are also describedbelow for the polymerisable LC material itself.

Also, the RM should not hinder the aligning capability of the polymerfilm. Thus, for example in a photoalignment layer the UV absorption ofthe RM should not interfere with that of the photoalignment layer.

Especially preferred are RMs comprising a mesogenic group that islinked, optionally by a spacer group, to one or more polymerisablegroup. Especially preferred are RMs with two similar, preferably twoidentical, terminal groups. These are beneficial because their symmetricstructure allows their alignment to be influenced by the alignmentlayer, whereas non-symmetrical RMs are ‘surfactant-like’ and so have apropensity for homeotropic alignment over planar alignment. Compoundscomprising two or more polymerisable groups (multireactive ormultifunctional compounds), like e.g diacrylate or triacrylate RMs, arepreferred because they contain two or three polymerisable groups forpossible reaction into a subsequent layer and can be synthesised to havesimilar terminaly substituted groups.

Especially preferred are RMs of formula I-VI above.

The term ‘polar group’ in formula I-VI means a group selected from F,Cl, CN, NO₂, OH, OCH₃, OCN, SCN, an optionally fluorinated carbonyl orcarboxyl group with up to 4 C atoms or a mono- oligo- or polyfluorinatedalkyl or alkoxy group with 1 to 4 C atoms.

The term ‘unpolar group’ in formula I-VI means an alkyl group with 1 ormore, preferably 1 to 12 C atoms or an alkoxy group with 2 or more,preferably 2 to 12 C atoms.

The polymerizable or reactive groups P¹, P² and P³ in formula I-VI arepreferably selected from a vinyl group, an acrylate group, amethacrylate group, an oxetane group or an epoxy group, especiallypreferably an acrylate group.

P¹, P² and P³ in one compound can be identical or different. Preferablythey are identical.

x and y in formula I-VI can be identical or different. Preferably theyare identical. Another preferred embodiment of the present inventionrelates to compounds wherein x and y are different. Another preferredembodiment relates to a mixture comprising at least two compounds , eachhaving identical groups P¹⁻³, Z¹⁻⁶, L¹⁻⁶ and r, but wherein x and y aredifferent.

Especially preferred are compounds of formula I-VI wherein L¹⁻⁶ areselected from F, Cl, CN, CH₃, C₂H₅, OCH₃, OC₂H₅, COCH₃, COC₂H₅, COOCH₃,COOC₂H₅, CF₃ and OCF₃.

Further preferred are compounds of formula I-VI wherein Z¹ and Z² areselected from —COO—, —OCO—, —CH₂CH₂— or a single bond.

Examples of suitable and preferred RM materials are:

wherein P¹, P², x, y, L¹ and L² are as defined above. In formula Ia P¹and P² are preferably acrylate, L¹ and L² are preferably H, CH₃ or OCH₃,and x and y are preferably integers from 3 to 6.

The compound of formula Va is disclosed for example in WO 97/14674. Someof the compounds of formula III are disclosed for example in JP2002-348319.

The thickness of an alignment layer according to the present inventionis preferably from 15 nm to 50 nm.

The LC films provided on the alignment layer, or formed on the alignmentlayer as part of the inventive laminate, are preferably prepared from apolymerisable LC material by in-situ polymerisation. In a preferredmethod of preparation a polymerisable LC material is coated onto thealignment layer acting as a substrate, oriented into the desiredorientation and subsequently polymerised for example by exposure to heator actinic radiation as described for example in WO 01/20394, GB2,315,072 or WO 98/04651.

Preferably the alignment layer is rubbed unidirectionally before beingcoated with the LC material.

If a polymerisable LC material is used, it preferably comprises one ormore polymerisable chiral or achiral mesogenic or liquid crystallinecompounds. It is preferably a mixture comprising one or morepolymerisable compounds having one polymerisable group (monoreactive)and one or more polymerisable compound having two or more polymerisablegroups (di- or multireactive).

In another preferred embodiment the polymerisable LC material comprisesup to 20% of a monoreactive non-mesogenic compound with onepolymerisable functional group. Typical examples are alkyl acrylates oralkyl methacrylates with alkyl groups of 1 to 20 C atoms.

In another preferred embodiment the polymerisable LC material used forthe preparation of the low crosslinked film does not contain compoundshaving more than two polymerisable groups.

In another preferred embodiment the polymerisable LC material used forthe preparation of the low crosslinked film is an achiral material, i.e.it does not contain chiral compounds.

The polymerisable compounds and polymerisable mesogenic compoundsreferred to above and below are preferably monomers.

The non-polymerisable compounds include for example additives likesurfactants, catalysts, sensitizers, stabilizers, chain-transfer agents,inhibitors, lubricating agents, wetting agents, dispersing agents,hydrophobing agents, adhesive agents, flow improvers, defoaming agents,deaerators, diluents, reactive diluents, colourants, dyes or otherauxiliaries.

In case of high crosslinked LC films it is also possible to add up to20% of a non-mesogenic compound with two or more polymerisablefunctional groups to the polymerisable LC material alternatively or inaddition to the di- or multireactive polymerisable mesogenic compoundsto increase the degree of crosslinking. Typical examples for direactivenon-mesogenic monomers are alkyl diacrylates or alkyl dimethacrylateswith alkyl groups of 1 to 20 C atoms. Typical examples for multireactivenon-mesogenic monomers are trimethylpropane trimethacrylate orpentaerythritol tetraacrylate.

Polymerizable mesogenic mono-, di- and multireactive compounds used forthe present invention can be prepared by methods which are known per seand which are described, for example, in standard works of organicchemistry such as, for example, Houben-Weyl, Methoden der organischenChemie, Thieme-Verlag, Stuttgart.

Examples of suitable polymerizable mesogenic compounds that can be usedas monomers or comonomers together with the compounds according to thepresent invention in a polymerizable LC mixture, are disclosed forexample in WO 93/22397, EP 0 261 712, DE 195 04 224, WO 95/22586, WO97/00600 and GB 2 351 734. The compounds disclosed in these documents,however, are to be regarded merely as examples that shall not limit thescope of this invention.

Examples of especially useful chiral and achiral polymerizable mesogeniccompounds (reactive mesogens) are shown in the following lists whichshould, however, be taken only as illustrative and is in no way intendedto restrict, but instead to explain the present invention:

In the above formulae, P is a polymerisable group, preferably an acryl,methacryl, vinyl, vinyloxy, propenyl ether, epoxy, oxetane or styrylgroup, x and y are identical or different integers from 1 to 12, A is1,4-phenylene that is optionally mono-, di- or trisubstituted by L¹, or1,4-cyclohexylene, u and v are independently of each other 0 or 1, Z⁰ is—COO—, —OCO—, —CH₂CH₂—, —CH═CH—, —C═C— or a single bond, R⁰ is a polargroup or an unpolar group, Ter is a terpenoid radical like e.g. menthyl,Chol is a cholesteryl group, L, L¹ and L² are independently of eachother H, F, Cl, CN or an optionally halogenated alkyl, alkoxy,alkylcarbonyl, alkylcarbonyloxy, alkoxycarbonyl or alkoxycarbonyloxygroup with 1 to 7 C atoms, and r is 0, 1, 2, 3 or 4. The phenyl rings inthe above formulae are optionally substituted by 1, 2, 3 or 4 groups L.

The term ‘polar group’ in this connection means a group selected from F,Cl, CN, NO₂, OH, OCH₃, OCN, SCN, an optionally fluorinated alkycarbonyl,alkoxycarbonyl, alkylcarbonyloxy or alkoxycarbonyloxy group with up to 4C atoms or a mono- oligo- or polyfluorinated alkyl or alkoxy group with1 to 4 C atoms. The term ‘unpolar group’ means an optionally halogenatedalkyl, alkoxy, alkycarbonyl, alkoxycarbonyl, alkylcarbonyloxy oralkoxycarbonyloxy group with 1 or more, preferably 1 to 12 C atoms whichis not covered by the above definition of ‘polar group’.

For the preparation of cholesteric LC films with helically twistedstructure, the polymerisable LC material preferably comprises one ormore achiral polymerisable mesogenic compounds and at least one chiralcompound. The chiral compound can be selected from non-polymerisablechiral compounds, like e.g. conventional chiral dopants, orpolymerisable chiral compounds, all of which can be mesogenic ornon-mesogenic.

Suitable polymerisable chiral compounds are for example those shown inthe above list. Further suitable chiral polymerisable compounds are e.g.the commercially available Paliocolour® materials (from BASF AG,Germany).

Suitable chiral dopants can be selected e.g. from the commerciallyavailable R- or S-811, R- or S-1011, R- or S-2011, R- or S-3011, R- orS-4011, R- or S-5011, or CB 15 (from Merck KGaA, Darmstadt, Germany).Very preferred are chiral compounds with a high helical twisting power(HTP), in particular compounds comprising a sorbitol group as describedin WO 98/00428, compounds comprising a hydrobenzoin group as describedin GB 2,328,207, chiral binaphthyl derivatives as described in WO02/94805, chiral binaphthol acetal derivatives as described in WO02/34739, chiral TADDOL derivatives as described in WO 02/06265, andchiral compounds having at least one fluorinated linkage group and aterminal or central chiral group as described in WO 02/06196 and WO02/06195.

The polymerisable material is preferably dissolved or dispersed in asolvent, preferably in an organic solvent. The solution or dispersion isthen coated onto the substrate, for example by spin-coating or otherknown techniques, and the solvent is evaporated off beforepolymerisation. In most cases it is suitable to heat the mixture inorder to facilitate the evaporation of the solvent.

The polymerisable LC material may additionally comprise a polymericbinder or one or more monomers capable of forming a polymeric binderand/or one or more dispersion auxiliaries. Suitable binders anddispersion auxiliaries are disclosed for example in WO 96/02597.Especially preferred, however, are LC materials not containing a binderor dispersion auxiliary.

In another preferred embodiment the polymerisable LC material comprisesan additive that induces or enhances planar alignment of the liquidcrystal material on the substrate. Preferably the additive comprises oneor more surfactants. Suitable surfactants are described for example inJ. Cognard, Mol.Cryst.Liq.Cryst. 78, Supplement 1, 1-77 (1981).Particularly preferred are non-ionic surfactants, very preferablyfluorocarbon surfactants, like for example the commercially availablefluorocarbon surfactants Fluorad FC-171® (from 3M Co.) or Zonyl FSN®(from DuPont) or the compounds described in GB 2 383 040.

The polymerisation of the RMs in the alignment layer as well as thepolymerisation of a LC material coated onto said alignment layer ispreferably carried out according to the methods described above andbelow.

Generally, polymerisation is preferably achieved by exposing it toactinic radiation. Actinic radiation means irradiation with light, likeUV light, IR light or visible light, irradiation with X-rays or gammarays or irradiation with high energy particles, such as ions orelectrons. Preferably polymerisation is carried out by photoirradiation,in particular with UV light. As a source for actinic radiation forexample a single UV lamp or a set of UV lamps can be used. When using ahigh lamp power the curing time can be reduced. Another possible sourcefor photoradiation is a laser, like e.g. a UV laser, an IR laser or avisible laser.

Polymerisation is carried out in the presence of an initiator absorbingat the wavelength of the actinic radiation. For example, whenpolymerising by means of UV light, a photoinitiator can be used thatdecomposes under UV irradiation to produce free radicals or ions thatstart the polymerisation reaction. UV photoinitiators are preferred, inparticular radicalic UV photoinitiators. As standard photoinitiator forradical polymerisation for example the commercially available Irgacure®or Darocure® series (all from Ciba Geigy AG) can be used, whereas incase of cationic photopolymerisation the commercially available UVI 6974(Union Carbide) can be used.

The polymerisable LC material can additionally comprise one or moreother suitable components such as, for example, catalysts, sensitizers,stabilizers, chain-transfer agents, inhibitors, co-reacting monomers,surface-active compounds, lubricating agents, wetting agents, dispersingagents, hydrophobing agents, adhesive agents, flow improvers, defoamingagents, deaerators, diluents, reactive diluents, auxiliaries,colourants, dyes or pigments.

It is also possible to add one or more chain transfer agents to thepolymerisable material in order to modify the physical properties of thepolymer film. Especially preferred are thiol compounds, such asmonofunctional thiol compounds like e.g. dodecane thiol ormultifunctional thiol compounds like e.g. trimethylpropanetri(3-mercaptopropionate), very preferably mesogenic or liquidcrystalline thiol compounds. When adding a chain transfer agent, thelength of the free polymer chains and/or the length of the polymerchains between two crosslinks in the inventive polymer film can becontrolled. When the amount of the chain transfer agent is increased,the polymer chain length in the obtained polymer film is decreasing.

The alignment layers according to the present invention are particularlyuseful as alignment layers or substrates for the preparation of LCfilms, in particular polymerised LC films.

The laminates according to the present invention are useful as opticalelements like polarisers, compensators, circular polarisers or colourfilters in liquid crystal displays or projection systems, as decorativeimage, for the preparation of liquid crystal or effect pigments, andespecially as reflective film with spatially varying reflection colours,e.g. as multicolour image for decorative, information storage orsecurity uses, such as non-forgeable documents like identity or creditcards, banknotes etc.

The laminates according to the present invention can be used in displaysof the transmissive or reflective type. They can be used in conventionalLCDs, in particular those of the DAP (deformation of aligned phases) orVA (vertically aligned) mode, like e.g. ECB (electrically controlledbirefringence), CSH (colour super homeotropic), VAN or VAC (verticallyaligned nematic or cholesteric) displays, MVA (multi-domain verticallyaligned) or PVA (patterned vertically aligned) displays, in displays ofthe bend mode or hybrid type displays, like e.g. OCB (opticallycompensated bend cell or optically compensated birefringence), R-OCB(reflective OCB), HAN (hybrid aligned nematic) or pi-cell (π-cell)displays, furthermore in displays of the TN (twisted nematic), HTN(highly twisted nematic) or STN (super twisted nematic) mode, in AMD-TN(active matrix driven TN) displays, or in displays of the IPS (in planeswitching) mode which are also known as ‘super TFT’ displays. Especiallypreferred are VA, MVA, PVA, OCB and pi-cell displays.

The examples below serve to illustrate the invention without limitingit. In the foregoing and the following, all temperatures are given indegrees Celsius, and all percentages are by weight, unless statedotherwise.

EXAMPLE 1

The reactive mesogen (1)

was added in different concentrations to a pre-imidized solution (AL1054of JSR Co.) and spincoated (3,000 RPM;30s) onto clean glass slides.Immediately after spin-coating the films were dried (100° C., 60 s) andimidized by baking at 180° C. for 90 minutes. The resulting polyimidefilms were rubbed with a velvet cloth (22 cm rub length) to providealignment layers for subsequent RM layers.

The polymerisable LC mixture M1 was formulated as follows:

M1: (1) 39.4% (2) 24.6% (3) 24.6% (4) 9.7% Irgacure651 1.0% FluoradFC171 0.6% Irganox1076 0.1%

Irgacure651® is a photoinitiator, Irganox1076® a stabilizer, both beingcommercially available (Ciba AG, Basel, Switzerland). FC171® is anon-ionic fluorocarbon surfactant (from 3M Co.).

A solution of polymerisable mixture M1 (50%) in xylene was deposited byspincoating onto the different rubbed polyimide layers and subsequentlyphotopolymerised (20 mWcm², 60s, N₂) to give a polymerised LC film.

The adhesion of the polymerised LC film to the polyimide layers wastested using the Scotch #610 tape test. The 610 tape was applied overthe RM film and removed sharply. The adhesion was deemed to pass if noneof the film was removed. Each film was tested 5 times. The results aresummarized in table 1:

TABLE 1 % of (1) in polyimide 0 1 5 10 Alignment¹⁾ good good poor poor³⁾Summary²⁾ fail pass pass pass ¹⁾Alignment Quality of M1 on Polyimide²⁾Summarized result of 610 Tape test ³⁾Scattering Film

The results show that adding a small amount of reactive mesogen offormula I to the polyimide precursor promotes adhesion between thepolymerised LC layer and the polyimide film, whilst giving clear,transparent, highly oriented films.

Comparison Example

The experiment as described in Example 1 was repeated using 1% of HDDA(hexanediol diacrylate) and TMPTA (trimethylolpropane triacrylate),respectively, instead of compound (1). The resulting polyimide filmsshowed phase separation of the acrylate materials and were opticallyscattering.

1. Alignment layer suitable for aligning liquid crystal (LC) molecules,characterized in that it comprises at least one reactive mesogen (RM) inmonomeric, oligomeric or polymeric form.
 2. Alignment layer according toclaim 1, characterized in that it comprises less than 50% by weight ofRMs.
 3. Alignment layer according to claim 1, characterized in that theRM(s) is(are) present in monomeric or oligomeric form in the alignmentlayer after its preparation.
 4. Alignment layer according to claim 1,characterized in that it is obtainable from a precursor materialcomprising at least one reactive mesogen (RM).
 5. Alignment layeraccording to claim 1, characterized in that it is a solvent processedfilm.
 6. Alignment layer according to claim 1, characterized in that itis a polyimide film.
 7. Alignment layer according to claim 6,characterized in that it is a polyimide film of the general formula A


8. Alignment layer according to claim 1, characterized in that it is asolvent processed cellulose based film.
 9. Alignment layer according toclaim 1, characterized in that it is a triacetate cellulose (TAC) ordiacetate cellulose (DAC) film.
 10. Alignment layer according to claim1, characterized in that it is a command layer comprising one or morecompounds selected from photochromic compounds, isomerisable compounds,chromophores and dyes, wherein changes of the chemical structure and/orthe orientational direction of these compounds induce a specificalignment of an LC material coated onto said layer.
 11. Alignment layeraccording to claim 10, characterized in that said compounds are selectedfrom derivatives of azobenzene, stilbenes, spiropyran, spirooxadines,α-hydrazono-β-ketoesters, cinnamate, retinylidene, chalcone, coumarins,benzylidenephthalimidines, benzylideneacetophenones, diphenylacetyleneor stilbazoles.
 12. Alignment layer according to claim 1, characterizedin that the RMs are selected of the following formulae

wherein P¹, P² and P³ are independently of each other a polymerisablegroup, Z¹ and Z² are independently of each other, —O—, —S—, —CO—, —COO—,—OCO—, —O—COO—, —OCH₂—, —CH₂O—, —CH₂CH₂—, —C═C—, —CH═CH—COO—,—OCO—CH═CH— or a single bond, Z³ and Z⁴ are independently of each other—COO—, —OCO—, —CH₂CH₂—, —CH₂O—, —OCH₂—, —CH═CH—, —CF═CF—, —C═C— or asingle bond, Z⁵ and Z⁶ are independently of each other —O—, —COO—,—OCO—, —CH₂CH₂—, —CH₂O—, —OCH₂— or a single bond, Y¹ and Y² areindependently of each other a polar group, R¹ and R² are independentlyof each other an unpolar alkyl or alkoxy group, A, B, C and D areindependently of each other 1,4-phenylene that is optionally mono- di ortrisubstituted by L¹, L², L³, L⁴, L⁵, L⁶ or 1,4-cyclohexylene, L¹, L²,L³, L⁴, L⁵ and L⁶ are independently of each other H, F, Cl, CN or anoptionally halogenated alkyl, alkoxy, alkylcarbonyl, alkoxycarbonyl oralkoxycarbonyloxy group with 1 to 7 C atoms. r is 0, 1, 2, 3 or 4, x andy are each independently an integer from 1 to 12, z is 1, 2 or 3,g¹,g²,g³ and g⁴ are independently of each other a single bond, —O—,—COO—or —OCO—,.
 13. Alignment layer according to claim 12, characterizedin that the RMs are selected of the following formulae

and the alignment layer is a TAC or DAC film.
 14. Alignment layeraccording to claim 1, characterized in that the precursor materialcomprises 0.5 to 4% by weight of RMs.
 15. Polymer precursor as definedin claim
 4. 16. Use of an alignment layer according to claim 1 assubstrate and/or alignment layer of liquid crystal (LC) materials. 17.Laminate comprising an alignment layer according to claim 1 and a filmcomprising polymerised or crosslinked LC material.
 18. Method ofpreparing a laminate by providing a layer of a polymerisable LC materialonto an alignment layer according to claim 1, optionally aligning the LCmaterial into uniform orientation, and polymerising or crosslinking theLC material.
 19. Use of a precursor material, alignment layer orlaminate according to claim 1 in optical, electrooptical, informationstorage, decorative and security applications.
 20. Optical component ordevice comprising at least one precursor material, alignment layer orlaminate according to claim
 1. 21. Liquid crystal display comprising atleast one alignment layer or laminate according to claim 1 or acomponent comprising the same.